The present invention relates to a fiber light source configured to emit light which is substantially insensitive to weapons level radiation, and which is particularly useful as a light source for an interferometric fiber optic gyroscope (IFOG).
Interferometric fiber optic gyroscopes (IFOG""s) are gyroscopes in which a light source is used to produce a signal representative of the rotational state of a moving craft in which the gyroscope is located.
Prior to the present invention, conventional wisdom suggested that a light source for an interferometric fiber optic gyroscope should have a power output with as wide a spectral width as possible. Hence, the power output of a light source for an interferometric fiber optic gyroscopes has tended to be centered about a mean wavelength in the range of about 1540 nm to 1570 nm, but may also have significant power output centered about lower wavelength(s).
However, Applicants have discovered that with an interferometric fiber optic gyroscope (IFOG), a light source with power output centered about a mean wavelength in the range of 1540 nm to 1570 nm can be severely attenuated when exposed to weapons level radiation threat such as gamma or neutron radiation threat. Such attenuation can significantly affect the gyroscope""s mean operating wavelength and the detected light level. Hence, an explosion or other event that exposes the interferometric fiber optic gyroscope to weapons level gamma or neutron radiation can significantly affect the gyroscope""s scale factor and random noise performance.
According to the present invention, Applicants have departed from conventional wisdom in interferometric fiber optic gyroscopes by their discovery that there is a mean wavelength range, and a preferred mean wavelength, at which an interferometric fiber optic gyroscope can perform its desired function, and in a manner whereby the light source is substantially unaffected by weapons level radiation threat such as gamma or neutron radiation. By xe2x80x9csubstantially unaffectedxe2x80x9d, it is meant that a substantial amount of power output from the light source is centered about a mean wavelength and does not drift by more than a predetermined amount when exposed to weapons level radiation. The amount of power output centered about the mean wavelength, and the amount of drift that can be tolerated, and still remain xe2x80x9csubstantially unaffectedxe2x80x9d by weapons level radiation may vary somewhat, depending upon the craft with which the light source is associated.
The present invention provides a new and useful fiber light source, configured to produce light with a mean wavelength which is substantially unaffected when exposed to weapons level radiation threat.
According to a preferred embodiment, the fiber light source is configured to produce a light output with a mean wavelength in the range of about 1520 to 1540 nm, and preferably substantially centered about a mean wavelength of about 1532 nm.
Moreover, according to the present invention, the light source is configured to be substantially unaffected by weapons level radiation when measured by one of a pair of criteria. By one criteria, the light source is configured to produce a light output shaped such that more than 50%, preferably at least 85%, and most preferably at least 99% of power is within a mean wavelength range of 1515 nm to 1545 nm, and is substantially centered about a mean wavelength (preferably 1532 nm) within that mean wavelength range. By another criteria, the light source is configured such that when exposed to weapons level radiation the output of the light source has a mean wavelength drift of less than 500 ppm (parts per million), preferably less than 150 ppm, and most preferably less than 15 ppm.
A fiber light source according to the invention comprises a light generator (e.g. a pump laser) and a material (preferably a length of fiber optic material) which is configured to receive light from the light generator and re-emit the light in the desired mean wavelength range. The configuration of the length of fiber optic material will depend on the specific properties of the fiber, but with the preferred fiber material (i.e. a length of Erbium Doped Fiber), the length of fiber would be designed to re-emit light in the predetermined mean wavelength range.
According to a preferred embodiment, an interferometric fiber optic gyroscope includes a fiber light source comprising a 980 nm pump laser and a length of Erbium Doped Fiber (EDF) configured to receive the light from the pump laser and to re-emit the light with a mean wavelength which is substantially unaffected by weapons level radiation threat. The length of EDF receives the 980 nm light from the pump laser, and re-emits the light at a longer mean wavelength which is useful in an interferometric fiber optic gyroscope. Specifically, the length of the EDF optimizes the mean wavelength of the re-emitted light to produce power output in a range of about 1515 to 1545 nm and substantially centered about a wavelength in that range. Preferably, the power output is substantially centered about a mean wavelength of about 1532 nm. According to the preferred embodiment, with a 980 nm pump laser, the EDF would have a length of about 5 to 15 meters, to re-emit the light in the foregoing mean wavelength range. The particular length of the EDF may be determined by the particular characteristics of the EDF used. Additionally, in order to produce re-emitted light with virtually no attenuation (i.e. at least 99% of output power centered about a mean wavelength of about 1532 nm, and less than 15 ppm mean wavelength drift) when exposed to weapons level radiation, the re-emitted light is preferably filtered (e.g. by band filter grating).
Further features of the present invention will be apparent from the following detailed description and the accompanying drawings.